1. Field of the Invention
The present invention relates generally to the field of fuel cells and, more particularly to methodology and apparatus for controlling the supply of reactant fluids to the cells and the purging of reaction products and inert fluids from the individual cells and/or groups of cells of a cell stack. In particular the invention relates to methodology and apparatus for tailoring the supply of fuel to and removal of reaction products from cells so as to meet the individual demands of each respective cell and/or group of cells in a stack of cells.
2. The State of the Prior Art
Fuel cells are electromechanical devices that convert chemical energy in the form of fuel and an oxidant directly into electrical energy. Fuel cells are generally classified in accordance with the type of electrolyte (e.g., alkaline, phosphoric acid, solid polymer, molten carbonate, solid oxide) used to provide ionic conduction between an anode and a cathode. Useful fuel cells include PEM fuel cells, acidic fuel cells and alkaline fuel cells. In this regard, PEM is the abbreviation for polymer electrolyte membrane or proton exchange membrane, and such membranes are proton-conducting thus facilitating the use thereof as an electrolyte for transporting protons from the anode to the cathode of the cell. The present invention is generally applicable to each of these types of cells.
A fuel cell generally includes seven major components, namely, 1) an anode current collector acting as the negative terminal to conduct electrons away from the cell; 2) a fluid plate to distribute and supply fuel to the anode and provide electrical contact between the anode current collector and the anode; 3) an anode comprising a porous diffusion layer and a catalyst layer where fuel is oxidized (e.g. hydrogen fuel is oxidized to form protons and electrons); 4) an electrolyte to provide ionic conduction between the anode and the cathode, which in the case of a PEM fuel cell is a proton conducting membrane; 5) a cathode with similar structure to that of the anode where an oxidant is reduced (e.g., oxygen reacts with protons and electrons to form water); 6) a fluid distributor plate to distribute and supply oxidant to the cathode and provide electrical contact between the cathode and a cathode current collector; and 7) a cathode current collector acting as a positive terminal to conduct electrons back to the cell.
During operation of a fuel cell, a fuel such as hydrogen is caused to flow into the cell through a fuel inlet, and the fuel is distributed by the fuel distributor plate on the anode side of the fuel cell. Hydrogen diffuses from the fuel distributor plate through the anode diffusion layer to the catalyst layer in the anode, where it is oxidized to form protons and electrons. The electrons are conducted out of the anode, through the fuel distributor plate, to the anode current collector, and out of the cell to power an electrical device. The protons are transported across the PEM to the cathode.
An oxidant such as oxygen is caused to flow into the cell through an oxidant inlet, and the oxidant is distributed by the oxidant distributor plate on the cathode side of the cell. Oxygen diffuses from the oxidant distributor plate through the cathode diffusion layer to the catalyst layer in the cathode, where it reacts with the protons generated in the anode and electrons to form water. The electrons are conducted from the electrical device back to the cell and to the cathode via the cathode current collector and the oxidant fluid distributor plate. The product water produced at the catalyst layer in the cathode diffuses through the cathode diffusion layer to flow channels in the oxidant fluid distributor plate and is removed from the cell by the flow of excess oxidant and/or inert materials in the incoming oxidant fluid stream.
Fuel cells typically produce a voltage which varies up to about 1.0V. Accordingly, in order to generate greater voltages, a plurality of fuel cells are stacked in series to form a fuel cell stack. The cells of a given stack generally are arranged with a common inlet for the fuel (e.g., hydrogen or methanol and water), a common outlet for the fuel, a common inlet for the oxidant (e.g., oxygen or air), and common outlet for the oxidant. The fuel and oxidant for each cell of the stack is fed to the respective cells from these common inlets, and the product water, inert materials, excess fuel and/or excess oxidant from each cell of the stack is removed from the respective cells through these common outlets. To prevent buildup of product water, inert materials, excess fuel and/or excess oxidant in the cells, and to maintain appropriately high concentrations of all reactants in all cells, continuous or frequent purging of fuel and/or oxidant has generally been required in the past. This leads to low fuel and/or oxidant utilization, whereby large amounts of parasitic power are required to provide the necessary flows of fuel and/or oxidant to the cell stack. Moreover, it is essentially impossible to provide fuel cells such that the same have identical dimensions and morphological properties (e.g., porosity, tortuosity, wetting characteristics). Thus, the flow resistance properties of the cells of a given stack are not uniform during operation.
This nonuniformity in flow resistance among the cells of a stack results in nonuniform fluid flow into and through the cells leading to nonuniform cell-to-cell performance and non-optimal stack performance. Cells having so much flow resistance that incoming flow of fuel or oxidant is inappropriately restricted and/or that proper purging of product or inert fluids is prevented become starved for fuel and/or fluid oxidant resulting in poor performance or perhaps even total failure of the cell. Adding to the non-optimal performance is the danger of explosion. In a fuel stack, cells receiving insufficient reactants to support the electrical current being drawn from the stack may go into reverse, thus generating a potentially explosive mixture in the cell. For example, when a hydrogen-oxygen cell experiences reversal, hydrogen is generated in the cathode where oxygen is present and oxygen is generated in the anode where hydrogen is present, thus provided an explosive fluid mixture of hydrogen and oxygen in both compartments.
In an effort to address this problem, fuel cell stacks have been used only in low current density operations and/or the same have been designed in such a way to insure that cells with the highest flow resistance obtain a sufficient supply of new reactants and are adequately purged of product and inert fluids. The latter is often accomplished by using high flow rates of reactants; however, as explained above, the use of high reactant flow rates results in low fuel and oxidant utilization and high parasitic power consumption. To minimize fuel loss in such a case, a recirculation loop is sometimes used. Such a loop is discussed in U.S. Pat. No. 5,316,869. In this scheme, recirculation blowers or pumps are used to provide sufficiently high flow rates of reactant fluids through the cell stack to assure adequate flow of fresh reactants to and purging of product and inert fluids from each cell. To assure that the entire loop is not saturated with product or inert fluids, a purging line is generally incorporated in the loop. Such purging line may be designed to be open continuously to remove the fluid mixture at such a rate that the system is not saturated with product or inert fluids. Alternatively the purging line may be designed for opening only when reactant fluid concentrations drop below a certain set value or when concentrations of product and/or inert fluids exceed certain set values. In the alternative case, a fluid sensor may be used to detect the concentrations which are being monitored. Either purging process results in high fuel loss rate and high parasitic power consumption to power the recirculation system.
The problem of properly disposing of product and inert gas is further addressed in U.S. Re. Pat. No. 36,148. This patent describes a fuel stack arrangement wherein the cells are divided into groups of cells that, on both the fuel side and the oxidant side, have parallel gas feed within each group, but serial feed from group to group. That is to say, on both the cathode side and the anodes side, reactant fluid is fed to the cells of the first group of cells in parallel. On the anode side, the exhaust from the first group is collected and fed to the cells of a second group of cells in parallel. Flow on the anode side continues in like manner for as many groups as are included in the stack. On the cathode side, liquid water product is separated from the exhaust, and the remainder of the first group exhaust is collected and fed to the cells of the second group of cells in parallel. Flow and water product removal on the cathode side continues in like manner for as many groups as are included in the stack.
The sequential feeding of the exhaust from one group of cells to the inlet of another group of cells presents a number of disadvantages as follows:
1) The fluid feed composition varies from group to group as the fluid traverses the stack. For example, when air is used as an oxidant, the oxidant containing feed stream becomes leaner in oxygen and richer in nitrogen as the fluid flows from one group to the next. Consequently, the cells of downstream groups have relatively poorer performance than the cells of upstream groups. The same is true on the fuel side of the stack.
2) travel path from stack inlet to stack outlet is longer and the amount of reactant fluid flowing through each cell group includes the amount needed for subsequent cell groups. As a result, the cell stack has a high pressure drop and requires a great deal of parasitic power to pump the fluid.
3) Since the cells of a given group are arranged for parallel fluid flow, nonuniformity among the individual cells results in nonuniform flow through the cells of a group. This problem has been discussed above.